Many automated stage light fixtures are fitted with an arc lamp which is the source of light for the fixture. These lamps, commonly called High Intensity Discharge (HID) sources, produce light by continuously discharging a plasma arc through a high pressure mixture of mercury vapor, noble gases and the evaporated salts of rare earth elements. When excited by the plasma arc, this mixture creates a small-volume light emitter with high luminous efficiency. The color and quality of the light emitted is primarily determined by the mixture of these elements and is typically similar to daylight with a Correlated Color Temperature (CCT) of about 6000 Kelvins.
Often these automated luminaires are used in conjunction with other lighting instruments in a television studio or on a film set. It then becomes desirable to adjust the color temperature of the HID source to match that of the other lighting instruments so that the different light sources will be rendered to look the same by the film or television camera. Typically the conventional lights have a tungsten filament source which produces light with a CCT of 3200 Kelvins. Often spotlights and other sources are used that have other, and sometimes higher color temperature beams. Therefore, the desired color temperature of the automated luminaire is dependent on the particular shot the camera is taking and on the other lighting instruments used in that particular shot.
In addition to color temperature, a second and equally important quality of the light emanating from the instrument is its Color Rendering Index (CRI). The illuminating beam's CRI is a measure of how well balanced its spectrum is compared to that of natural daylight or more specifically, compared to a black body radiator at a similar color temperature. Light with a high CRI renders all colors faithfully while that with a low CRI, like poor quality fluorescent illumination, can give false impressions of colors. Therefore having a luminaire with adjustable CCT while maintaining a high CRI is very beneficial as the cameras will render their subjects' color faithfully. This is especially important when the subject is human skin since we are all extremely sensitive to the appearance of skin tones. Light with a low CRI illuminating an individual can make them look ill.
Existing adjustable color temperature correcting systems for automated luminaires are capable of only lowering the color temperature of the light. Furthermore, the CRI of the adjusted light usually deteriorates as the color temperature is adjusted which is inherent in the design of the filtration system. The filtration is typically an optical thin film applied to a glass wheel where a portion of the wheel intersects the light source beam inside the luminaire.
The filter 100 is typically spatially patterned to produce a density gradient that runs circumferentially around the wheel as shown in FIG. 1A running from an open area at 110 to a low density area at 115, to higher density areas at 120. This allows the saturation of the filter coating to vary around the wheel. The rotational position of the wheel then controls the color temperature of the beam exiting the luminaire.
At the beginning of the gradient, the patterning completely removes all of the filter material so it has no effect on the natural color temperature of the beam. This is called the “open” position 110 of the filter wheel. At the end of the gradient or “full-in”, position 130 most or all of the filter material is left on the wheel so that the color temperature of the filtered beam is the desired minimum CCT, usually around 3000 Kelvins. In between, over the area 120 the varying density filter gradient causes a changing ratio between filtered and unfiltered light passing through the wheel and therefore a change in CCT of the beam. The changing CCT of the light beam with wheel position is illustrated in the 1933 CIE chromaticity diagram of FIG. 1B as a series of points connected by the dotted line. The color temperature of several points along the line are noted beginning with the “open” temperature of the unfiltered beam of 5600K and ending with the “full-in” CCT of 2450K.
Note that the locus of filtered color correction points is straight between “open” 210 and “full-in” 220 indicating that a change in saturation has been caused by the filtration, the saturation of the color point being affected by the patterning density while the hue imparted by the filter material remains constant. Note also that the CRI of the light decreases as the locus of filtered light diverges away from the Planckian Locus or Black Body Curve; the Planckian Locus being the locus of white points all having a perfect Color Rendering Index of 100.